Vertical Cavity Surface-Emitting Lasers (VCSELs) are currently being considered for use in many high speed communication systems. A VCSEL may be viewed as a laser having mirrors constructed from alternating layers of material having different indices of refraction. These lasers are better suited for the fabrication of arrays of lasers for displays, light sources, optical scanners, and optical fiber data links.
To increase the bandwidth of an optical communication network, multiple fibers are used to transmit information in parallel. Each fiber must be driven by a separate laser. One method for driving the multiple fibers is to couple an array of VCSELs to a fiber optic ribbon cable.
Electrically, each VCSEL is equivalent to a light emitting diode that is driven by a semiconductor driver. To provide the highest possible bandwidth to the fibers, the lasers must be driven as fast as possible. Accordingly, N-type driving circuits are needed. However, to obtain an N-type driving circuit, conventional VSCSEL designs require that the VCSEL be constructed on a p-doped compound-semiconductor substrate.
A VCSEL array is electrically equivalent to an array of photodiodes, which are connected to the underlying substrate. Since the substrate is a semiconductor, the diodes are connected in common. This leaves only the top layer of the diode for connection to the individual driving circuits. Hence, to use N-type driving circuits, the top of the diode must be n-doped. Accordingly, the bottom mirror layers are constructed from p-doped semiconductor layers.
Construction of a VCSEL on a p-doped substrate leads to a device that has inherently poorer performance than devices constructed on an n-doped substrate. This difference in performance is the result of the higher diffusivity of the p-type dopants. It is also more difficult to provide good current confinement in the top n-type mirror layer.
The VCSEL is constructed by depositing a series of layers of semi-conducting materials over the substrate. Each deposition step subjects the device to high temperatures. When the bottom mirror layers are constructed from p-type materials, the dopants from the layers already deposited to form the bottom mirror diffuse upwards into the n-type doped regions that are fabricated on top of the light emitting layer and which form the top mirror.
Accordingly, it is preferable to construct the device on an n-doped substrate, as the n-type dopants exhibit lower diffusion, and hence, do not contaminate the p-type layers fabricated on top of the light emitting layer. However, this is incompatible with the n-drive requirement.
Broadly, it is the object of the present invention to provide an improved VCSEL design.
It is a further object of the present invention to provide a VCSEL array in which the individual VCSELs may be driven by either p-drive or n-drive circuitry.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.